A time-of-flight (TOF) mass spectrometer has become a powerful analytical tool that provides a simple and easy method to obtain exact mass measurements. Its characteristic features are high sensitivity, theoretically infinite mass-range, and rapid measurements. These features give the TOF mass spectrometer a great advantage over other mass spectrometers, such as the quadrupole, ion trap, magnetic sector-type mass spectrometers, and the like. However, TOF mass spectrometers need special design features to attain the resolving power necessary for accurate mass analysis.
A TOF mass spectrometer comprises at least three major components: an ion source, a free-flight region, and an ion detector. In the ion source, molecules from the sample are converted to volatile ions, usually by high-energy bombardment, each ion being characterized by its mass-to-charge ratio, or m/z.
Following ionization, ions of appropriate polarity are accelerated to a final velocity by an electric field and enter the free-flight region. This acceleration and extraction imparts a kinetic energy to each of the ions, and each ion acquires a final velocity after acceleration that is inversely proportional to the square root of its mass. Accordingly, lighter ions have a higher velocity than heavier ions.
During free-flight, ions of different masses separate as a consequence of their different velocities. After traversing the free-flight region, the ions arrive at the ion detector component. The time taken by an ion to traverse this distance, known as the time-of-flight (TOF), may be used to calculate the mass of the ion. In this manner, a time-of-flight spectrum may be converted into a mass spectrum of the original sample.
Mass resolution of a TOF mass spectrometer is expressed as t/2Δt, where t is the total time of flight, which is given by the flight path length divided by the ion velocity, and Δt is the peak width measured at full width at half maximum (FWHM). Accordingly, extending the flight path length and minimizing the peak width is particularly effective to improve the mass resolution. However, elongation of a flight distance on a straight line results in enlargement of the device. The peak width depends on the broadening of the ion packet at the detector, especially along the velocity axis and the response time of the detector. Various ion optical techniques have been reported to minimize the peak width: space focusing; time-lag focusing; orthogonal acceleration; and an ion mirror or sector fields.
Most commercial TOF mass spectrometers are based on linear or reflectron ion optics. Their flight path lengths are often one to several meters and they depend on the size of the instrument. Thus, the method by which the flight path length can be increased to a length much greater than the instrument size is crucial for the improvement of mass resolution and mass accuracy.
Multi-turn TOF mass spectrometers using electrostatic sector fields have been proposed as a solution to increase flight length without drastically increasing instrument size. In this type of spectrometer, the flight path length is not restricted to the instrument size and can be increased because the ions revolve around a closed orbit. Multi-turn TOF mass spectrometers have been constructed: a large multi-turn TOF mass spectrometer comprising six electrostatic analyzers that produces an elliptical orbit; and a compact multi-turn TOF mass spectrometer consisting of four cylindrical electrostatic sectors, 16 electrostatic quadrupole lenses, and providing a figure-eight-shaped ion orbit having a shorter flight path length. The design was further compacted by utilizing four toroidal electrostatic sectors and four cylindrical electrostatic sectors.
A multi-turn TOF mass spectrometer has “overtaking” problems. Because ions having different mass (m) to charge (z) ratios revolve in the closed orbit repeatedly, the faster ions with smaller m/z values pass the slower ones with larger m/z values. As a result, ions with different m/z values do not arrive at an ion detector in the order of their m/z value. Certain complex mathematical treatments are then required to transform the deformed mass spectrum into one that is of the order of the m/z value. Some proposals to avoid such complex mathematical treatments include the use of a spiral trajectory so that ions travel for approximately fewer cycles along a helical ion trajectory or the use of complicated electrical gating to only let ions in a small mass range enter the multi-turn path.
Single path TOF spectrometers have followed two main design patterns to achieve reasonable mass accuracy when the ions have a spread in initial energy: “Reflectron” designs, in which electrodes arranged in a nearly linear fashion, turn the ion beam nearly back upon itself in a controlled fashion; and “Sector” designs, in which electrodes are sections of cylinders, toroids or similar shapes, turn the ion beam through a nearly circular arc with a specific angle.
To realize a high ion transmission, mass resolution and mass accuracy, broadening of the ion packet must be minimized not only along the velocity axis but also along the axes perpendicular to the velocity axis. Several sector designs have been proposed with focusing properties along these axes. These designs focus an input slit onto an exit slit. However, overall geometry of such designs is inconvenient. The output beam is at an odd angle relative to the input beam. Most designs require a large vacuum chamber relative to the length of the ion path. In addition, every sector must be paired with a straight section of a specific length immediately before and after each sector. One multi-turn sector design included special ion optics to compensate for straight sections that were not the optimal length for that geometry.
Accordingly, it is desirable to provide apparatus and methods for performing TOF mass spectrometry with improved mass resolution and/or the sensitivity of mass spectra, in which the output ion beam is parallel to the input ion beam, there is more flexibility in positions of the source and detector, the entire geometry folds into a very compact volume, and certain higher-order aberrations cancel when the ion beam makes turns.